At 8 p.m. Iraqi time four years ago Tuesday, U.S. combat sappers in tanks and bulldozers gashed holes in a sandy berm on Kuwait's northern border and began the invasion of Iraq. The air war had begun about 15 hours earlier with a "decapitation strike" aimed at Iraqi President Saddam Hussein. The strike failed; Saddam hadn't been where U.S. informants said he'd be. It wouldn't be the last intelligence failure of this war.

Within a few hours, Marine 2nd Lt. Terrel Shane Childers, 30, of Powell, Wyo., was shot dead near an oil pumping station near the town of Rumeila by an Iraqi fighter riding in the back of a pickup truck. Lt. Childers became the first American to die from hostile fire in the Iraq war. He wouldn't be the last.

Over the next few days, more than a 100,000 American troops poured into Iraq from Kuwait. Loudspeakers on psychological operations trucks boomed a heavy metal sound track: Drowning Pool's "Let the Bodies Hit the Floor." Guns 'n Roses' "Welcome to the Jungle." Reporters and photographers embedded with U.S. units uplinked, for the first time ever, real-time coverage of ground combat. Fire fights and sandstorms, the crack of M-4 carbines, the thudding of 105 mm howitzers, the loud barking of M-240 machine guns. Oh, it was heady stuff, and the Iraqis didn't stand a chance.

"It is well that war is so terrible," said Robert E. Lee. "We should grow too fond of it." Each generation must re-learn that truth. For this generation, it is Iraq, not a particularly bloody war in historical context, not for Americans, anyway. Since Shane Childers was killed, 3,216 more Americans have died in Iraq - seven last weekend - and 23,400 have been wounded. About that same number of Americans were killed on each side in three days at Gettysburg. About half that number died on D-Day, and about twice that number died at Iwo Jima.

For most Americans, the measure of the tragedy of Iraq is not the absolute numbers of casualties, but the cause in which they were incurred. There were no weapons of mass destruction. There was no terrorist hotbed, not until we created fertile ground for it. There was no real chance of imposing a western-style democracy on a fractious pseudo-country with no tradition of self-rule but rather one of strong tribal and religious affiliations. Four years on, the United States finds itself spending $2.2 billion a week, to say nothing of a dozen or more lives every week, trying to bring order to a place that defies order.

Think of the words that had no meaning for most of us just four short years ago: Mahdi Army. Spider hole. Shaped charges. Sunni Triangle. Golden Mosque of Samarra. Stinger. CENTCOM. Cobra II. Wahabist. Sharia. Waterboard. SAPI plates. Up-armored Humvee. IED.

Think of the places of which we were blissfully unaware: Fallujah. Tall Afar. Abu Ghraib. Al Anbar province. Sadr City. The northern city of Mosul. The oil-rich city of Kirkuk. The Green Zone.